The discussion throughout this specification comes about due to the realisation of the inventors and/or the identification of certain prior art problems by the inventors.
There are many applications where modulated sine waves are passed through frequency selective circuits. Transmitters are one example where data modulated carriers are passed through filters and matching circuits before being transmitted by an antenna. These filter and matching circuits are often narrowband high Q in order to achieve good transmitter efficiency. High Q circuits however are not able to pass data modulated sine waves where the data rate is higher then the circuit bandwidth.
The inventors have realised that in the past, data has been transmitted to RFID tags by modulation of the excitation field. Generally pulse position modulation (PPM) is used, where 100% depth amplitude modulation of the interrogation field is used. The interrogation field is turned off for short pulse periods and this is detected by the tag's processing circuitry.
The inventors have realised that to achieve high data rates whilst maintaining the transmission of power, these pulses must be short and the duty cycle must be low. Typically a duty cycle of approximately 10% is used where the pulses are 9 μs long and the average time between pulses is around 75 μs.
Short pulses such as these have a bandwidth much wider than the original data bandwidth. Accordingly, to pass the inherently broad band of the PPM interrogation signal, both the interrogator and tag antenna must have a bandwidth significantly wider than the original data bandwidth. Consequently the interrogator and the tag antennae must have a relatively low Q factor to transmit and receive the data. However, antennas with a low Q factor are relatively inefficient and will operate with an efficiency lower than would otherwise be required for practical commercial applications of this technology as only low data rates are possible.
The inventors have further realised that additionally, PPM produces relatively high level modulation product side bands. For passive tags, a stronger excitation field is required to compensate for a less efficient antenna. Emission regulations must also be kept in mind and these place restrictions on side band transmissions including modulation products that can be transmitted. This places restrictions on the maximum excitation field strength that can be used. In order to reduce the sideband levels the modulation depth can be reduced. Modulation depths of between 10% and 30% have been proposed. An example of such systems is provided by ISO14443 an internationally recognised RFID system.
For these small modulation depths the tag voltage regulation circuits connected to the tag antenna will reduce the amplitude detected by the tag through the effect of amplitude compression. The stronger the interrogation field the greater the level of amplitude compression. However, amplitude compression of the PPM signal leads to a much reduced operating range for systems using PPM.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.
An object of the present invention is to provide an improved data transmission method and apparatus.
A further object of the present invention is to alleviate at least one disadvantage associated with the prior art.